1,970 research outputs found
Dynamics of the Nematic-Isotropic sharp interface for the liquid crystal
In this paper, we derive the sharp interface model of the nematic-isotropic
phase transition from the Landau-de Gennes theory by using the matched
asymptotic expansion method. The model includes the evolution equation of the
velocity and director field of the liquid crystal, the sharp interface and
Young-Laplace jump condition on the interface.Comment: 20 page
Theory and computation of directional nematic phase ordering
A computational study of morphological instabilities of a two-dimensional
nematic front under directional growth was performed using a Landau-de Gennes
type quadrupolar tensor order parameter model for the first-order
isotropic/nematic transition of 5CB (pentyl-cyanobiphenyl). A previously
derived energy balance, taking anisotropy into account, was utilized to account
for latent heat and an imposed morphological gradient in the time-dependent
model. Simulations were performed using an initially homeotropic
isotropic/nematic interface. Thermal instabilities in both the linear and
non-linear regimes were observed and compared to past experimental and
theoretical observations. A sharp-interface model for the study of linear
morphological instabilities, taking into account additional complexity
resulting from liquid crystalline order, was derived. Results from the
sharp-interface model were compared to those from full two-dimensional
simulation identifying the specific limitations of simplified sharp-interface
models for this liquid crystal system. In the nonlinear regime, secondary
instabilities were observed to result in the formation of defects, interfacial
heterogeneities, and bulk texture dynamics.Comment: first revisio
Field-driven dynamics of nematic microcapillaries
Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of
study for their unique electro-optical properties which have resulted in
various applications such as switchable (transparent/translucent) windows.
These composites are manufactured using desirable "bottom-up" techniques, such
as phase separation of a liquid crystal/polymer mixture, which enable
production of PDLC films at very large scales. LC domains within PDLCs are
typically spheroidal, as opposed to rectangular for an LCD panel, and thus
exhibit substantially different behaviour in the presence of an external field.
The fundamental difference between spheroidal and rectangular nematic domains
is that the former results in the presence of nanoscale orientational defects
in LC order while the latter does not. Progress in the development and
optimization of PDLC electro-optical properties has progressed at a relatively
slow pace due to this increased complexity. In this work, continuum simulations
are performed in order to capture the complex formation and electric
field-driven switching dynamics of approximations of PDLC domains. Using a
simplified elliptic cylinder (microcapillary) geometry as an approximation of
spheroidal PDLC domains, the effects of geometry (aspect ratio), surface
anchoring, and external field strength are studied through the use of the
Landau--de Gennes model of the nematic LC phase.Comment: 22 pages, 9 figures, Physical Review
Formation and kinetics of transient metastable states in mixtures under coupled phase ordering and chemical demixing
We present theory and simulation of simultaneous chemical demixing and phase
ordering in a polymer-liquid crystal mixture in conditions where isotropic-
isotropic phase separation is metastable with respect to isotropic-nematic
phase transition. It is found that mesophase formation proceeds by a transient
metastable phase that surround the ordered phase, and whose lifetime is a
function of the ratio of diffusional to orientational mobilities. It is shown
that kinetic phase ordering in polymer-mesogen mixtures is analogous to kinetic
crystallization in polymer solutions.Comment: 17 pages, 5 figures accepted for publication in EP
First-order phase transitions in two-dimensional off-lattice liquid crystals
We consider an off-lattice liquid crystal pair potential in strictly two
dimensions. The potential is purely repulsive and short-ranged. Nevertheless,
by means of a single parameter in the potential, the system is shown to undergo
a first-order phase transition. The transition is studied using mean-field
density functional theory, and shown to be of the isotropic-to-nematic kind. In
addition, the theory predicts a large density gap between the two coexisting
phases. The first-order nature of the transition is confirmed using computer
simulation and finite-size scaling. Also presented is an analysis of the
interface between the coexisting domains, including estimates of the line
tension, as well as an investigation of anchoring effects.Comment: 12 pages, 17 figures, submitted to J. Phys.: Condens. Matte
Colloidal particles at a nematic-isotropic interface: effects of confinement
When captured by a flat nematic-isotropic interface, colloidal particles can
be dragged by it. As a result spatially periodic structures may appear, with
the period depending on a particle mass, size, and interface
velocity~\cite{west.jl:2002}. If liquid crystal is sandwiched between two
substrates, the interface takes a wedge-like shape, accommodating the
interface-substrate contact angle and minimizing the director distortions on
its nematic side. Correspondingly, particles move along complex trajectories:
they are first captured by the interface and then `glide' towards its vertex
point. Our experiments quantify this scenario, and numerical minimization of
the Landau-de Gennes free energy allow for a qualitative description of the
interfacial structure and the drag force.Comment: 7 pages, 9 figure
Cellular solid behaviour of liquid crystal colloids. 1. Phase separation and morphology
We study the phase ordering colloids suspended in a thermotropic nematic
liquid crystal below the clearing point Tni and the resulting aggregated
structure. Small (150nm) PMMA particles are dispersed in a classical liquid
crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that
small colloid particles densely aggregate on thin interfaces surrounding large
volumes of clean nematic liquid, thus forming an open cellular structure, with
the characteristic size of 10-100 micron inversely proportional to the colloid
concentration. A simple theoretical model, based on the Landau mean-field
treatment, is developed to describe the continuous phase separation and the
mechanism of cellular structure formation.Comment: Latex 2e (EPJ style) EPS figures included (poor quality to comply
with space limitations
- …